Moisture-absorbing materials are used in a variety of everyday household items such as bath towels, paper towels, diapers, sponges, etc. The design goal of each of these items is to maximize absorption for a given surface area without any concern for how the item grows or expands as a result of such absorption.
In other specialized applications of moisture-absorbing materials, it may be desirable to harness the expansion of the moisture-absorbing material to perform work. For example, a mechanical water sensor described in U.S. Pat. No. 6,182,507, uses compressed cotton balls constrained in an open frame as a means to absorb water and expand where the force of expansion is used to move a piston. However, compressed cotton balls do not provide a reliable means of moisture absorption in harsh underwater environments and, therefore, are not reliable as a means of producing work when subjected to immersion in such environments. This is because the compressed cotton balls rely on surface absorption of moisture for its expansion. However, high-levels of naturally-occurring impurities and man-made pollutants often found in underwater environments can cover the surface area of the cotton thereby impeding the absorption of water.
More recently, a water sensing actuator described in U.S. Pat. No. 6,561,023, discloses a moisture-absorbing material that is based on a fibrous cellulosic material having anisotropic moisture-absorbing properties such that its dried-in strain is greatest along one axis thereof. A powder material coats, and can be mixed with, the cellulosic material. The powder material is inert with respect to the cellulosic material and initiates a chemical reaction when exposed to water such that a product of the chemical reaction is water. While this material was found to absorb water and expand even in impure water environments, the direction of such expansion was somewhat unpredictable thereby requiring a housing encasing the material to control such expansion.